TOKYO—Shares of Japan Display Inc. sank 18% Thursday, just a day after it announced a much bigger downward revision to its earnings outlook than analysts had expected.

After the market closed Wednesday, Japan Display said it now predicted a loss of ¥10 billion ($95 million) in the year ending in March, instead of a ¥26.8 billion net profit previously expected. It blamed fierce market competition and delays of shipments to Apple Inc., whose business accounts for nearly one-third of Japan Display’s revenue.